Direct sequence code division multiple access (DS-CDMA) allows signals to overlap in both time and frequency so that CDMA signals from multiple users simultaneously operate in the same frequency band or spectrum. In principle, a source information digital data stream to be transmitted is impressed upon a much higher rate data stream generated by a pseudo-random noise (PN) code generator. This combining of a higher bit rate code signal with a lower bit rate data information stream “spreads” the bandwidth of the information data stream. Accordingly, each information data stream is allocated a unique PN or spreading code (or a PN code having a unique offset in time) to produce a signal that can be separately received at a receiving station. From a received composite signal of multiple, differently coded signals, a specifically PN coded information signal is isolated and demodulated by correlating the composite signal with the specific PN spreading code associated with that specific information signal. This inverse de-spreading operation compresses the received signal to permit recovery of the original data signal and at the same time suppresses interference from other users.
In addition to receiving signals transmitted from several different transmitting information sources, a receiver may also receive multiple, distinct propagation paths of the same signal transmitted from a single transmitter source. One characteristic of such a multipath channel is an introduced time spread. For example, if an ideal pulse is transmitted over a multipath channel, the received corresponding signal appears as a stream of pulses, each pulse or path having a corresponding different time delay, as well as different amplitude and phase. Such a complex received signal is usually called the channel impulse response (CIR). Multipaths are created in a mobile radio channel by reflection of the signal from obstacles in the environment such as buildings, trees, cars, people, etc. Moreover, the mobile radio channel is dynamic in the sense it is time varying because of relative motion affecting structures that create the multipaths. For a signal transmitted over a time varying multipath channel, the received corresponding multiple paths vary in time, location, attenuation, and phase.
The existence of multiple paths, however, may be used to advantage in a CDMA system using signal diversity combining techniques. One advantage concerns signal fading which is a particular problem in mobile communications. Although each multipath signal may experience a fade, all of the multipaths usually do not fade simultaneously. Therefore, a diversity-combined signal output from a CDMA receiver is not adversely affected by a temporary fade of one multipath.
A CDMA receiver in accordance with the present invention employs a multipath search processor that searches for and identifies the strongest multipaths along with their corresponding time delays. A RAKE demodulator captures most of the received signal energy by allocating a number of parallel demodulators (called RAKE “fingers”) to the strongest multipath components of the received multipath signal as determined by the multipath search processor. The outputs of each of the RAKE fingers are diversity-combined after corresponding delay compensation to generate a “best” demodulated signal that considerably improves the quality and reliability of communications in a CDMA cellular radio communications system.
The multipath search processor, (sometimes referred to herein as simply “searcher”) estimates the channel impulse response of a complex received signal in order to extract the relative delays of various multipath components. The searcher also tracks changing propagation conditions resulting from movement of the mobile station or some other object associated with one of the multipaths to adjust the extracted delays accordingly.
More specifically, the channel impulse response of a received multipath signal is estimated within a certain range of path arrival times or path arrival delays called a “search window.” This window is defined by the number of spreading code phases which should be searched to cover the maximum expected delay spread. All signals detected within the search window form the delay profile, but only those signals originated from the transmitter belong to the channel impulse response. The remaining received signals in the delay profile are noise and interference. When the signals forming the delay profile are represented by their respective powers and delays, the delay profile is called power delay profile (PDP).
The channel impulse response is estimated very frequently so that delay variations of the radio channel can be tracked. In particular, the position of the channel impulse response within the search window frequently changes because of movement of the mobile station or other object motion as well as from frequency mismatch of the PN sequence generators used at the transmitter for spreading and at the receiver for de-spreading. As a result, the position of the search window must be updated and adjusted to keep the channel impulse response in the middle of the search window. The update time should be small enough so that the delay variations of the radio channel can be tracked.
The position of the channel impulse response within the search window changes as a result of motion of the mobile station (and the resulting change of propagation delay) as well as the frequency mismatch of the transmitter and receiver PN sequence generators. A window tracking unit (WTU) adjusts the position of the search window to keep the channel impulse response within the search window by tracking the time-varying delay between the mobile station PN code and the reference PN code of the base station.
It is desirable to make the search window adaptation robust in order to minimize the influence of noise, interference, and fading. It is particularly difficult to keep the channel impulse within the search window under certain propagation conditions including, for example, slow fading conditions, e.g., 0.5 km/h–3 km/h, as well as fast fading conditions, e.g., 250 km/h–500 km/h. Fading is a problem because when a path in the search window “disappears” because of a fade, the natural response of the WTU is to adjust the position of the search window, often significantly, assuming that the path no longer exists. But that assumption is typically wrong because the faded path very often reappears. If the search window is adjusted too rapidly, it may be badly misaligned when the faded path reappears. Another area of concern is the need to use fine-tuned decision threshold(s) in search window tracking, which add to the complexity and delay of the tracking adjustment process. These difficulties are overcome with the present invention.
Thus, it is an object of the present invention to adapt the search window position in order to maintain accurate alignment between the estimated channel impulse response and the search window under a wide range of propagation conditions.
It is also an object of the present invention to provide a robust search window delay adjustment procedure that minimizes the influence of noise, interference, and fading, and in particular, takes into account a likelihood that faded paths may reappear.
It is a further object of the invention to avoid the use of one or more decision thresholds to accomplish accurate window tracking adjustment.
The present invention provides a search window delay tracking procedure for use in a multipath search processor of a CDMA radio receiver. A channel impulse response is estimated for a received signal containing plural paths, each path having a corresponding path delay. A search window defines a delay profile that contains: (1) the plural multipath components of the received signal forming the channel impulse response (CIR), and (2) noise and interference signals at delays where the transmitted multipath components do not exist. A mean or average delay is calculated for the estimated channel impulse response, and an error is determined between the mean CIR delay and a target delay position, e.g., the center of the CIR search window. An adjustment is made to reduce that error so that the target position and the mean CIR delay are aligned. A Doppler effect associated with the received signal is taken into account in determining the adjustment signal.
In particular, a Doppler frequency is determined for the received signal and used to determine a maximum rate at which the search window can be moved when it is adjusted. In other words, if a path fade occurs, the search window is restricted in how fast it can shift the search window in responding to the faded path. The maximum search window shifting rate is related to the maximum Doppler frequency of the received signal. One way of implementing this is to formulate a minimum dwell time (MDT) for the search window using a maximum Doppler frequency estimate of the received signal. The MDT is the minimum time the search window should stay in its current position before being adjusted to a new position. The MDT should not be too long because the PN generators may drift to the point where a search window adjustment is necessary. However, the MDT should not be too short that an adjustment is made before a faded path will reappear. In other words, the restricted shift rate or minimum dwell time ensures that the search window tracking unit on the one hand does not correct (or overcorrect) too soon and on the other hand does not correct too late.
In a preferred example embodiment, the search window adjustment signal is determined by averaging the error between the mean delay and the target delay position using the minimum dwell time. For example, if the minimum dwell time is five signal transmission frames, and the error is determined once per frame, the five errors are summed and divided by five to arrive at an average error. If desired, upper and/or lower limits can be used for the averaging period, e.g., for high Doppler frequencies, low Doppler frequencies, or both. In addition, the adjustment amount can be limited if desired.
One example application of the present invention is to a radio base station that includes plural cells, each one of the cells having one or more directive antennas receives a signal from a mobile station that contains multiple paths. Each path has a corresponding delay. A multipath search processor at the base station includes plural channel estimators, one corresponding to each of the plural cells. Each channel estimator generates a delay profile within a search window containing the actual channel impulse response as well as noise and interference. A path selector in the multipath search processor selects paths with strongest signals from the delay profiles generated by each channel estimator and outputs a selected channel impulse response made up of the corresponding delay and power for each selected path. A window tracking unit maintains alignment of channel impulse response and a target position of the search window. A demodulator demodulates the selected paths and combines the demodulated paths into a combined received signal. The window tracking unit adjusts the search windows in the channel estimators, taking the maximum Doppler frequency of the received signal into account to maintain accurate alignment, as well as adapt the delays for the selected paths according to the search window adjustment.